1. Technical Field
The present invention relates in general to the field of semiconductor devices. More particularly, the present invention relates to the field of semiconductor devices incorporating a system memory on a single integrated circuit substrate. Still more particularly, the present invention relates to the field of semiconductor devices incorporating a system memory on a single integrated circuit substrate and a means for modifying instructions for operating the semiconductor devices with minimal production lead time.
2. Description of the Related Art
In recent years, a system large-scale integration (LSI) configuration has been developed that enables logic circuits such as central processing units (CPU), multiprocessing units (MPU), and memory cells of a dynamic random access memory (DRAM) and peripheral circuits to be implemented on a single integrated circuit substrate. The system LSI configuration has many notable advantages. First, there is no need for a conventional external memory element to be included outside the system LSI configuration. This reduces both the manufacturing costs and the size of the integrated circuit substrate. Additionally, an external data (or address) bus coupling an integrated circuit to an external DRAM via an input/output (I/O) pin is not required, since the system LSI configuration integrates the external DRAM. The number of data (or address) buses coupled via I/O pins is no longer a limitation since such connections are made internally on the single integrated circuit substrate. The system LSI configuration is also effective in shortening the trace length of each data bus and improving the overall performance of the system. Finally, the system LSI configuration is a preferred configuration of various peripheral devices of a data processing system, such as a computer system.
If the system LSI configuration is implemented as a semiconductor device utilized as a control element for peripheral devices such as a hard disk drive of a computer system, required instructions for operating the logic circuits must be written to the semiconductor device in advance. Generally, a non-volatile memory device, such as a mask read-only memory (ROM), a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM) is utilized for storing required program instructions or microcodes. It should be readily apparent to those skilled in the art that there exists a trade-off between the controllability and price of the non-volatile memory device. The mask ROM is less costly, but cannot be modified once a set of instructions have been written. Consequently, the instructions must be finalized before the mask ROM is manufactured. The PROM is also less expensive and enables rewriting of a program after the program is written. Both the EPROM and the EEPROM are the more expensive options, although both allow a program to be erased after the program is written therein. A different set of instructions can then be written to either the EPROM or the EEPROM. In recent years, a type of EEPROM, called a flash EEPROM has been developed, so that large capacity and cost effective ROMs that enable rewriting of data electrically are now widely available.
A control element for peripheral devices such as a hard disk drive of a computer system may be developed to include a recording element that enables the rewriting of microcodes. The recording element is required due to the difficulty of microcode development. Early generation microcodes frequently contain program errors or bugs, despite the best efforts of microcode developers. Furthermore, user specifications or requirements might be altered during the development of the microcodes. When a mask ROM is employed in a semiconductor configuration, a few months are required before a reliable final microcode version can be implemented due to the production lead time required for a mask change of the semiconductor device to the completion of the semiconductor device. Considering the rapid generational improvements of hard disk drive technology in recent years, such a lengthy production lead time would exceed a permissible range for timely product releases. Therefore, careful consideration should be given to the employment of an EPROM or EEPROM as a recording element for microcodes. Now that the costs of a flash EEPROM are reduced due to an increase of demand and an increase of storage capacity, some of the controlling elements with logical circuits and a flash EEPROM implemented on a single integrated circuit substrate have been developed and are now in general use.
Ideally, the most favorable topology would be the implementation of a system LSI configuration with a DRAM and a flash EEPROM on a single integrated circuit substrate as a controlling element. However, the system LSI configuration with a DRAM and a flash EEPROM is difficult to implement utilizing conventional manufacturing techniques. Because of this difficulty, the flash EEPROM and the system LSI are mounted separately when the system LSI configuration utilizes a flash EEPROM. A serial flash EEPROM is employed to reduce the necessary number of input/output connections.
Furthermore, a microcode is first stored in a serial type external flash ROM on the assumption that the microcode is often modified in the initial stages of development. When the microcode is finalized, a mask ROM is formed in the system LSI. If the microcode is stored in the mask ROM in the mass-production stage and no problem arises from the manufacturing technique, the abovementioned procedure is considered to be advantageous in both technical and manufacturing cost aspects.